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I've heard it said various times before that humans can't breathe pure oxygen, or at least not for very long, before it would kill us. But during a discussion on Worldbuilding, the difference between air-breathing and water-breathing came up. It made me wonder: what would happen to a fish if it had pure oxygen, rather than the ordinary gas mixture of air, dissolved in its water? Would it be safe, or have the same oxygen toxicity problems as humans?

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    $\begingroup$ It can't be pure oxygen if it is dissolved in water. Also there are not tiny air bubbles in water it is dissolved gasses. You need to look up what dissolved means. $\endgroup$
    – John
    Jan 18, 2019 at 16:57
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    $\begingroup$ ...huh? I know exactly what "dissolved" means, and I didn't say anything at all about "air bubbles," tiny or otherwise. I asked what would happen if the gas dissolved in the water was pure oxygen rather than air. $\endgroup$ Jan 18, 2019 at 17:32
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    $\begingroup$ Pure oxygen is not dangerous because of a lack of other gasses it is dangerous because of the concentration, the amount of oxygen dissolved in the water will not be effected by whether the oxygen started pure or not. $\endgroup$
    – John
    Jan 18, 2019 at 18:14
  • $\begingroup$ Tropical fish are typically shipped with oxygen enrichment of the gas volume of the plastic bags. The oxygen is between about 30 and 90% depending on technique. There is also tranquilizer in the water. Local pet shops also inflate the bags ;I don't know if they use oxygen or compressed air. $\endgroup$ Jan 18, 2019 at 21:52

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It won't be safe. Typically there is a limit to how much oxygen can be in the water due to the physics of solubility. However, there are some circumstances where the water can be temporarily supersaturated with oxygen. For instance, near a waterfall, dam/thermal discharge, algae bloom etc.

If a fish were to remain there, they may contract gas bubble disease. Mortality of young salmon and trout has been observed after 3 days at 120% oxygen saturation.

You can read more about it at --- Fondriest Environmental, Inc. “Dissolved Oxygen.” Fundamentals of Environmental Measurements".

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  • $\begingroup$ the saturation rate of oxygen will not be affected by whether the oxygen starts pure or mixed with air. So this does not actually answer the question. $\endgroup$
    – John
    Aug 22, 2023 at 12:42

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